Hitherto, there are proposed a variety of techniques of reducing noise components included in audio signals in order to make voice and sound easy to be heard. For instance, a method of spectral deduction is known as a technique of analyzing inputted audio signals in real time and further controlling signals in frequency ranges thereby to reduce the noise components.
By deducting the noise spectrum estimated on the basis of spectral signals divided by Fourier transformation from the audio signals containing the noise components, the method of spectral deduction is a method of making it easy to listen to the audio signals after Fourier transformation.
In the method of spectral deduction, the presence or absence of audio signals is determined on the assumption that the noise components are stationary elements, and a noise model is specified. Additionally, it is also possible to adjust the noise spectrum for deduction, corresponding to the intensity of signal component and the kind of signal (vowel, consonant, fricative, etc.).
In the method of spectral deduction, the minimum mean squared error method etc. is used to estimate the noise spectrum. In the minimum mean squared error method, generally, there are executed repeated computations to minimize a square error mean of different signals in a specific observing zone, requiring a sizable amount of computation.
In addition, it is required to extract stationary noise components in the method of spectral deduction. For the application field, it is suitable in the utilization of sound receiving signals of a microphone in a mobile telephone in a situation where nearly-constant circumferential noises are produced, video-conference call system and so on.
As an application of this method, for the purpose of reducing the amount of computation furthermore, there is disclosed a technique of restraining by-productive noise components generated when a signal component changes greatly (quantization noise based on an error generated by the adjustment in the level of spectrum component and the deduction of an estimated error) in Patent Document No. 1.
In Patent Document No. 1, there is disclosed a technique of controlling the reduction amount of the noise components with respect to each conversion zone corresponding to the characteristic of signals and further conducting the above reduction amount rapidly with reference to a table established to reducing the amount of calculations in advance.
Further, in order to restrain an occurrence of quantization noise in response to a transient signal as possible, Patent Document No. 2 discloses a technique of temporally smoothing a control amount of noise components controlled with respect to each conversion zone and a technique of applying weighting on each analytical zone.
In prior art, there is known a voice enhancing technology to enhance a predetermined frequency band in the audio signals in order to enable even the aged have a fading sense of hearing to catch the sound clearly. This technology is utilized in audio reproducing devices, hearing aids and so on.
For instance, in order to prevent high frequency component from being emphasized too much when enhancing the voice and also prevent the sound volume from being increased too much in spite of a high input sound pressure, Patent Document No. 3 discloses a technique of analyzing inputted audio signals and further providing an appropriate gain adjustment based on the analysis result thereby to improve the clearness of sound.
In addition, similarly to Patent Document No. 3, in order to avoid a phenomenon that emphasizing of only a specific frequency band in enhancing the voice causes the balance of original sound to collapse thereby increasing the difficulty in hearing, Patent Document No. 4 discloses a technique of comparing a sound volume set in an audio signal reproducing device with the levels of input signals and further supplying them with a gain corresponding to the input sound pressure thereby to emphasize the audio signals so as to enable vocal or speech to be clearly audible even at a small volume and also emphasize the audio signals appropriately while maintaining the balance of original sound at a middle or high volume.